The present invention relates, in particular, to a reduction gear for an electric power steering apparatus in which a worm shaft is supported movably in a rotation axis direction through an elastic body, and a movable amount adjustment method for adjusting a movable amount of the worm shaft of the reduction gear for an electric power steering apparatus.
In the electric power steering apparatus in which a motor is driven in compliance with a steering angle of a steering wheel and a rotation output of the motor is transmitted to an output shaft of a steerage mechanism by being decelerated by a worm gear mechanism, there is known a structure elastically supporting the worm shaft formed with a worm in an axial direction in order to set a suitable backlash between tooth faces of the worm and a worm wheel which constitute a worm gear mechanism (for example, refer to Patent Document 1 and Patent Document 2). According to this structure, since the worm meshing with the worm wheel becomes movable in the axial direction, even in a case where an excess torque is exerted on the worm wheel, a collision between the tooth faces of the worm and the worm wheel is relieved by a movement of the worm in the axial direction, so that it becomes possible to reduce an abnormal noise by the collision of the tooth faces. Incidentally, in the above Patent Documents, as a member for elastically supporting the worm in the axial direction, there is disclosed an elastic body such as conical spring and O-ring each of which is provided between a flange part of the worm and a bearing.
Patent Document 1
JP-A-11-43062
Patent Document 2
JP-A-11-171027
By the way, in the structure in which the worm shaft is supported through the elastic body like the above related art, there is considered a case where a dispersion in movable amount of the worm occurs owing to a dimension error and an incorporation error of an individual such as the elastic body. In such a case, an intended moving amount of the worm shaft is not ensured, or the backlash of the bearing becomes liable to occur because a pressure application to the bearing is insufficient. As a result, there may occur a disadvantage that a feeling of a steering wheel operation is deteriorated, or the abnormal noise is generated. In a case where a load exceeding an intended load is applied to the elastic body owing to the dispersion in movable amount of the worm, a problem occurs from a viewpoint in durability of the elastic body.
Further, in the structure in which the worm shaft is supported through the elastic body like the above related art, since a sliding motion occurs between the elastic body and a spring seat with an expansion/contraction of the elastic body, wear particles generated from the sliding parts enter into meshing parts of the worm and the worm wheel, so that there arises also a problem that the durabilities of the worm and the worm wheel are deteriorated. Further, conversely, if foreign matters enter between the elastic body and the spring seat, there is a case where an expansion/contraction operation of the elastic body is hindered, so that there is a fear that an appropriate movable amount of the worm becomes unobtainable.
Accordingly, in the structure in which the worm shaft is supported through the elastic body, it becomes important to assure the appropriate movable amount of the worm in solving the above disadvantage and problems Additionally, it is desirable that the appropriate movable amount of the worm at this time is within such a range that no elastic body is deformed exceeding an allowable maximum deformation amount.